Planning of the JUICE/JANUS camera observations during the first ever Lunar-Earth Gravity Assist

Kurzfassung

The Jupiter Icy Moons Explorer (JUICE) spacecraft, launched in April 2023, will reach the Jovian system in July 2031 and will conduct an extensive science campaign of the Jupiter system, with a strong focus on Jupiter itself and of its icy Galilean satellites. During its cruise phase the spacecraft performed a double gravity assist maneuver (Moon–Earth) in August 2024, followed by a "farewell" observational sequence on 9 September 2024. These events constitute the Lunar Earth Gravity assist (LEGA) campaign. JANUS, the high resolution optical camera of JUICE, used this opportunity to perform an imaging campaign under realistic illumination, viewing geometry and thermal conditions that closely mimic the forthcoming Jupiter operations. JANUS acquired a total of 461 images in full frame, lossless mode, covering the complete set of 13 spectral filters (340–1080 nm). This paper summarizes the end‑to‑end workflow from the definition of scientific and engineering constraints, through the design and ground‑segment validation of the observation sequences, to the acquisition, processing and preliminary analysis of the LEGA data. The campaign demonstrated that a rapid thermal‑pre‑warm strategy using the spacecraft survival heaters can bring the JANUS optics to the required −20 °C thermal stability significantly limiting the power draw but preserving image quality. Exposure times were limited by a 1/4‑pixel smearing criterion; nevertheless, exposure times ranging from 0.22 ms to 163 ms yielded adequate signal‑to‑noise ratios across all filters, and full 13‑filter colour sequences were successfully obtained at high ground‑track speeds. A series of images acquired with increasing compression ratios showed no perceptible degradation, establishing a realistic data‑volume budget for the science phase. Simultaneous measurements by MAJIS, NavCam and the external Earth‑observation satellites enabled a three‑way cross‑calibration that will improve the absolute radiometric accuracy of JANUS. The LEGA experience shows that extending payload operations during cruise‑phase flybys dramatically enhances calibration quality, cross‑instrument synergy and scientific return. These lessons will directly inform the planning of the remaining JUICE Earth flyby windows (September 2026 and January 2029) and provide a best‑practice template for the Jupiter phase of the mission.